It always amazes me how people are so free to judge other's situations.
It's great that your web app only costs $10/month but others may have web apps that are more computationally intensive e.g. video processing or ML inference etc or simply can't join everything they need at runtime.
And it's great that you're willing to deny those 50k users a day access to your service when that cheap VPS inevitably falls over. But others may be monetising that traffic and will want a HA solution so their revenue isn't impacted.
All of those add complexity and cost to an architecture.
It's great that your web app only costs $10/month but others may have web apps that are more computationally intensive e.g. video processing or ML inference etc or simply can't join everything they need at runtime.
And it's great that you're willing to deny those 50k users a day access to your service when that cheap VPS inevitably falls over. But others may be monetising that traffic and will want a HA solution so their revenue isn't impacted.
All of those add complexity and cost to an architecture.